I don't know much about besides his offensive gifts, but reading the scouting report on DraftExpress regarding his defense:
Although he's far from being a finished product on this end of the floor, Porzingis also shows interesting potential defensively, evidenced by his 1.5 steals and 2 block averages per-40 minutes. His combination of size, length and mobility gives him impressive versatility on the pick and roll, allowing him to hedge out past the 3-point line and still recover in time to make a play in the paint for example. He's also agile enough to switch and stay in front of smaller players if called upon, giving his team the type of positional flexibility every coaching staff in the NBA covets greatly these days.
Besides the potential he shows as an athletic “stretch four,” Porzinigis also possesses another coveted attribute in his shot-blocking ability. The 6.6 blocks he averaged per-40 minutes this past summer at the U18 European Championship is the highest rate of any player in our database since 2012 by a wide margin. His 2.9 blocks per-40 is also the third best rate in the past fifteen seasons in Spain among players under the age of 20 (better than what current NBA per-minute shot-blocking leader Serge Ibaka averaged at the same age for example).
Porzingis' length, mobility and instincts give him great potential in general on this end of the floor. Despite his size, he has no problem covering most power forwards on the perimeter, as he does an excellent job of staying in front thanks to his terrific lateral quickness, and uses his length to contest shots effectively. He is also very adept at hedging screens well beyond the 3-point line, still being able to recover quickly enough to make a play at the rim, as he understands the concept of verticality quite well. Additionally, he uses his quickness to get in the passing lanes prolifically, averaging 1.7 steals per-40 minutes.
Maybe Okafor has been more scrutinized since he's a more known commodity, but outside his gifted post game he has glaring weaknesses defensively. I want the best player picked with the most potential and if the front office believes it's most likely will be Porzingis they should take him and not go with the safest pick.